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Title:Understanding the evolution of compositional zoning in San Juan Volcanic Field ignimbrites using iron isotope ratios
Author(s):Finlayson, Valerie A.
Advisor(s):Lundstrom, Craig C.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
igneous petrology
San Juan Volcanic Field
aplite, granodiorite
stable isotopes
compositional zoning
trace elements
Abstract:The San Juan Volcanic Field (SJVF) of southwestern Colorado is an erosional remnant of a large Tertiary aged calc-alkaline ignimbrite flare up. Previous work has led to detailed characterization of the volcanic stratigraphy and temporal evolution of many large calderas. Ignimbrites include both compositionally zoned rhyolitic to dacitic eruptions as well as monotonous intermediates. However, the petrogenetic origin of the volcanic field and the mechanisms of magma compositional evolution within magma chambers prior to eruptions remain largely unknown. Here I present new iron (Fe) isotope ratio data along with 87Sr/86Sr ratios and major and trace elements for 37 samples from the SJVF with the goal of improved understanding of the origin of zoned ignimbrites. I sampled in detail the zoned 33.2 Ma Bonanza Tuff as well as two intrusive units from the Bonanza Caldera. I also sampled in detail the 27.55 Ma Carpenter Ridge Tuff and collected representative samples from other major eruptions: the 28.02 Ma Fish Canyon tuff, the 26.9 Ma Nelson Mountain tuff, and the 26.9 Ma Rat Creek tuff (all ages given are from Lipman and McIntosh, 2008). Detailed X-ray mapping and compositional analysis were performed; compositions range from andesite to rhyolite, and granodiorite to aplite. Fe was purified from dissolved whole rock samples and analyzed for isotopic ratios using a high resolution MC-ICPMS. Results for δ56Fe are consistent with previous work, which has shown δ56Fe values of igneous rocks hover around the mean mafic earth value of 0.09‰ and increase in the most evolved high silica compositions. δ56Fe in the Carpenter Ridge ignimbrites shows a systematic increase in δ56Fe over much of the compositional range (68% to 78% SiO2) but importantly, increases to higher δ56Fe (up to 0.26‰) at the highest silica contents (74% and higher). Similarly, Bonanza intrusive rocks also follow a trend of increasing δ56Fe with silica content. Granodiorite samples exhibit slightly increased values (<+0.15‰), while aplites have substantially higher δ56Fe (+0.54‰ to +0.78‰). In contrast, the Bonanza ignimbrites show much more complex behavior with no simple progression with either silica or stratigraphy. This is consistent with the observation that the Bonanza ignimbrite does not exhibit a simple compositional stratigraphy. Instead of basal rhyolites grading upward into dacites, a dacitic unit, comprised of a series of “flow units” tens of meters thick, is overlain by high-silica rhyolite with no compositional gradient between the two. δ56Fe of the dacites range from -0.02‰ to +0.35‰ while the rhyolitic sample has a δ56Fe of +0.10‰. δ56Fe values oscillate within the dacite stratigraphy. Greater values are found near the bases of flow units within the ignimbrite, grading into lower values near the tops. Typically, concentrations of some incompatible trace elements progressively change upward within flow units as well, while major elements do not change significantly. Based on observations in the Bonanza ignimbrite, I propose a “pulse” eruption model during which magma is emplaced in the chamber and erupted after a short residence time. A process such as thermal migration could produce the trace element and isotopic zonation observed in flow units without substantial major element differentiation occurring. The data presented here provide new insight into and evaluation of differentiation processes in ignimbrites, yet add more questions about how ignimbrite zoning develops. Further chemical data and models of small-scale thermal migration and traditional differentiation processes are needed to assess the origin of compositional zoning in ignimbrites.
Issue Date:2011-08-25
Rights Information:Copyright 2011 Valerie A. Finlayson
Date Available in IDEALS:2011-08-25
Date Deposited:2011-08

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